Wireless communication networks transmit communication signals in the downlink over radio frequency channels from fixed transceivers, known as base stations, to mobile user equipment (UE) within a geographic area, or cell. The UE transmit signals in the uplink to one or more base stations. In both cases, the received signal may be characterized as the transmitted signal, altered by channel effects, plus noise and interference. To recover the transmitted signal from a received signal, a receiver thus requires both an estimate of the channel, and an estimate of the noise/interference. The characterization of a channel is known as channel state information (CSI). One known way to estimate a channel is to periodically transmit known reference symbols, also known as pilot symbols. Since the reference symbols are known by the receiver, any deviation in the received symbols from the reference symbols (once estimated noise/interference is removed) is caused by channel effects. An accurate estimate of CSI allows a receiver to more accurately recover transmitted signals from received signals. In addition, by transmitting CSI from the receiver to a transmitter, the transmitter may select the transmission characteristics—such as coding, modulation, and the like—best suited for the current channel state. This is known as channel-dependent link adaptation.
Modern wireless communication networks are interference limited. The networks typically process transmissions directed to each UE in a cell independently. Transmissions to other UEs in the same cell are regarded as interference at a given UE—giving rise to the term mutual interference. One approach to mitigating mutual interference is multi-user multiple input / multiple output (MU-MIMO). With MU-MIMO, a signal to be transmitted to multiple users is formed jointly, and these transmissions are formed taking into account the interference that transmission to one user creates at all other users. To operate most effectively, a MU-MIMO transmitter requires information about the transmission channels to each UE. That is, the transmitter requires CSI. Note that both single-cell MU-MIMO techniques and multi-cell MU-MIMO techniques can benefit from the availability of CSI at the transmitter.
Even without MU-MIMO transmission, CSI at the network can solve one of the most fundamental problems plaguing current wireless system—the inaccuracy in channel-dependent link adaptation due to the network not being able to predict the interference experienced by the UEs (a problem closely related to the well-known flash-light effect). Once the network knows the CSI of bases near each UE, the network can accurately predict the SINR at each UE, resulting in significantly more accurate link adaptation.
Even though the advantages of direct CSI feedback are clear, the major issue with direct CSI feedback is overhead. Full CSI feedback requires a high bitrate to transmit the CSI from each UE to the network. Time-frequency uplink channel resources must be used to carry the CSI feedback on the uplink channel, making these resources unavailable for transmitting user data on the uplink—the CSI feedback transmissions are thus pure overhead, directly reducing the efficiency of uplink data transmissions. Conveying direct CSI feedback to the network without consuming excessive uplink resources stands as a major challenge of modern communication system design.
Digital loopback was recently proposed as an efficient means to deliver CSI to the network with reasonable overhead and with quite low complexity. See, e.g., U.S. Pat. No. ,8,208,397, filed Sep. 9, 2009, tilted “Efficient Uplink Transmission of Channel State Information,” assigned to the assignee of the present application and incorporated herein by reference in its entirety. In digital loopback, a UE transmits succinct, direct channel state information to the network without substantially increasing uplink overhead. The UE receives and processes reference symbols over a set of non-uniformly spaced sub-carriers, selected according to a scheme synchronized to the network. The frequency response for each selected sub-carrier is estimated conventionally, and the results are periodically quantized and transmitted to the network on an uplink control channel. This is referred to herein as persistent digital loopback. Based on the information transmitted by the UE on the uplink channel, the network is able to construct an estimate of the frequency response of the channel at all sub-carriers, with a certain fidelity for a given bitrate of CSI. Naturally, the higher the bitrate of the CSI, the higher the fidelity of the channel estimation at the network will be.